The cell (or packet) switching technique, also called ATM from the initials of the designation "Asynchronous Transfer Mode", is becoming increasingly important for switching digital flows at very high rates, such as the flows present in digital broadband integrated service networks. In this technique, the information relevant to the various services is organized into adjacent units with a fixed length (about 400 bits), called "cells", comprising an information field and a header field, carrying the connection identifier together with other service information.
Due to the rate and bandwidth requirements of integrated service networks, the use of optical technologies has increased interest in the realization of both transmission lines and switching function. At the present state of technology, however, optical technologies do not allow high processing capacities, and the optical switching systems require the use of an electrical control part (therefore with a high processing capacity) and of an optical transport part (with a high capacity of information flow throughput). Examples of these networks have been described, for example, in the European Patent Applications EP-A-0 224 244 and EP-A-0 411 562 and in the papers: "A Photonic Knockout Switch For High-Speed Packet Networks", presented by K. Y. Eng at the IEEE/IEICE Global Telecommunications Conference 1987, 15-18 Nov. 1987, Tokyo, Japan; "Optical Technology Application to Fast Packet Switching" presented by P. Cinato and A. de Bosio, at the Topical Meeting on Photonic Switching, 1-3 Mar. 1989, Salt Lake City, USA; "Electro-Optical switching node based on fast packet and frequency switching techniques", presented by A. de Bosio et al. at the International Switching Symposium, May 1990, Stockholm, Sweden.
Electronic control units however have operating speed limits which could prevent full exploitation of the optical part capabilities because they limit the bit rate on the input lines to the node.